The inkjet recording method is currently achieving rapid development in various fields, because the high-definition image can be recordable by relatively simple apparatus. The inkjet recording method is applied to various fields and the recording medium or the inkjet inks (hereinafter also simply referred to as an ink) are used just for each purpose. Recently, the recording speed is extremely increasing and the development of printer which has performances applicable for the light printing.
In the inkjet recording method, the image is formed by ejecting an inkjet ink from inkjet head and depositing these droplets on the recording medium. Low viscosity is required for the inkjet ink because droplets of inkjet ink have to be ejected stably from inkjet head. On the contrary, since color mixing phenomena so called bleed tends to occur by depositing plural of inkjet inks having different colors on the recording medium, thereby it is required to use inkjet inks having high viscosity so as to prevent bleed.
As one of the methods to solve this dilemma, an ultraviolet curable inkjet is put to practical use in which a photo curable ink is used. In this image forming method, balance between stable ejection and prevention of bleed is achieved by adding a photo curable monomer and a photo polymerization initiator in an ink and by curing the ink via irradiating light just after depositing on the recording medium. However this method still has other essential problems.
That is, photo-curing conditions are restricted by a polymerization inhibition. Polymerization systems are classified broadly into a radical polymerization and a cationic polymerization. Photo polymerization is inhibited by oxygen in a radical polymerization and by water in the air in a cationic-polymerization system. Therefore, in an inkjet method in which the dots by small droplets are formed by non-contact on a substrate, so as to resolve above inhibition factors, countermeasure is necessary such as using a large-sized ultraviolet irradiation apparatus with high illumination and high intensity for a radical polymerization system, and controlling humidity for a cationic-polymerization system.
Moreover, in case of the light printing use, printed image has to have high weather resistance. Since these inhibitions for curing cause worse weather resistance, an ink which has no curing inhibition and high weather resistance is desired.
As a general method of reducing oxygen inhibition in a radical polymerization, used is a method in which amine such as triethanol amine is added into ink in order to trap oxygen radical. Moreover, recently proposed is a method which uses a monomer having an amino group (for example, Patent documents 1 and 2), but the effect is restrictive.
Also proposed is a method which adopts a polymerization system which is insusceptible to oxygen inhibition. As the polymerization system which is insusceptible to oxygen inhibition, known are an ene-thiol polymerization system (for example, refer to Patent document 3) and a polymerization system using a charge transfer complex in which a monomer having excess electrons such as vinyl ether compound and an electron-poor monomer such as the maleimide compound are used in combination (CT polymerization system (Sonny Jonsson, et. al, Polymer Materials Sci. & Enginer. 1995 and 72, 470-472). However, since an ene-thiol polymerization system has an offensive odor peculiar to thiol, and used maleimide compound has poor solubility, it is not able to use as inkjet ink as it is.
As an attempt to increase a solubility of the maleimide compound used in CT polymerization system, proposed is a maleimide compound which has alkylene oxide in the central part (for example, refer to Patent document 4) or a maleimide compound derived from oleic acid dimer (for example, refer to Patent document 5). However, these compounds have too high molecular weight and viscosity of the ink becomes high, thereby it is difficult to eject this ink stably.
Since there is an attempts to eject an ink of CT polymerization system from an inkjet head by diluting with solvents and lowering viscosity (refer to Patent document 6), it is inadequate as inkjet ink because it cannot be fixed without dryer or it occurs head clogging caused by drying the solvent from head surface.